This invention relates to a heat-sensitive record material and particularly to a heat-sensitive record material which has improved water resistance and is adapted for a high speed and unremitting recording so that it may find its usefulness as a recording medium for information machines and instruments such as facsimiles, electronic computers and telex machines.
There is known a heat-sensitive record material comprising a base sheet having a color developing layer which includes finely divided particles of one of electron donating color forming materials (hereinafter referred as "color former") and finely divided particles of one of electron accepting reactant materials (hereinafter referred as "acceptor"). In such the heat-sensitive record material like this the above-mentioned two kinds of particles are, when at least one of them is melted or sublimated at an elevated temperature, brought into intimate contact with each other to develop a color.
The color developing layer of the above-mentioned heat-sensitive record material further includes as a binder, polyvinyl alcohol, methyl cellulose, hydroxylcellulose starch, gum arabic, gelatin, styrene maleic anhydride copolymer, acrylic acid resin, etc. Since those compounds are water-soluble binders, use of them involves a disadvantage that the color developing layer shows poor water resistance. Accordingly, when the color developing layer of the heat-sensitive record material is attacked by water, the coating layer is destroyed or soiled by dissolving. Another disadvantage involved is that the thermal head for applying heat to the heat-sensitive record material is stuck on the surface of the heat-sensitive record material. This "sticking" causes the recording to become discontinuous or intermittent and is liable to damage the thermal head.
Various attempts have been made to overcome the above-mentioned disadvantages. In many cases, those attempts involve new disadvantages and accordingly are not always successful to completely solve the problems. For example, Japanese Published Unexamined Patent Applications Nos. 49-32,646, 49-36,343 and 50-30,539 proposes to use compounds having at least two ethylene imine radicals or 1,2 epoxy ring as a water-resisting agent for the water soluble binder. The water resistance obtained with use of such compounds like these is unstable and unsatisfactory, because the cross linking reaction between the water-soluble binder and such the water-resisting agent cannot be proceeded with under such the low temperature drying condition as is required for the production of the heat-sensitive record material described. In addition, in some cases, the cross linking reaction makes the binder film heat-softenable with the result of making the color developing layer much sticky.
Another attempt was made to utilize metal compounds as a cross linking agent for a water soluble binder having carboxyl radicals, e.g. as disclosed in Japanese Published Unexamined Patent Application No. 52-145,228. Though this method does not make the binder film heat-softenable by the cross linking reaction, the water resistance qualities obtained are not unstable and unsatisfactory since the cross linking reaction cannot be proceeded with under such the low temperature drying conditions as required for the production of the heat-sensitive record material as well. The principal object of this invention is to provide a heat-sensitive record material in which the above-mentioned disadvantages are improved and which has good water resistance qualities and is adapted for a high speed and unremitting recording.
Other objects and advantages of the invention will be apparent from the following detailed description.